While downlink frame transmission period as proposed hereinbelow may be used in various kinds of wireless communications, a WLAN system will be taken as an exemplary system to which the present invention is applicable.
Standards for the WLAN technology have been developed as Institute of Electrical and Electronics Engineers (IEEE) 802.11 standards. IEEE 802.11a and b use an unlicensed band at 2.4 GHz or 5 GHz. IEEE 802.11b provides a transmission rate of 11 Mbps and IEEE 802.11a provides a transmission rate of 54 Mbps. IEEE 802.11g provides a transmission rate of 54 Mbps by applying Orthogonal Frequency Division Multiplexing (OFDM) at 2.4 GHz. IEEE 802.11n provides a transmission rate of 300 Mbps for four spatial streams by applying Multiple Input Multiple Output (MIMO)-OFDM. IEEE 802.11n supports a channel bandwidth of up to 40 MHz and, in this case, provides a transmission rate of 600 Mbps.
Since the above-described standards for the WLAN technology maximally use bandwidth of 160 MHz and support eight spatial streams, IEEE 802.11ax standardization is being discussed in addition to IEEE 802.11ac standard maximally supporting a rate of 1 Gbit/s.